Drag from fixed blade propellers, on sailboats with inboard auxiliary engines, causes an appreciable loss of speed when under sail with the motor off. Propellers which have been designed to reduce drag are known and they are generally of two main types. One type is to have the blades which fold together to reduce the area that is impinged on by the water while the vessel is moving. The other is known as "feathered" blades which rotate to a fore and aft position where the blades cut through the water to minimize friction.
The known folding propellers normally have only two blades which are caused to open by centrifugal force when the propeller is turning and they are closed by drag on the blades through forward motion of the vessel in the water when rotation of the propeller is stopped.
The folding blade propellers are of two types, one in which the blades are free to open and close independent of one another and the other, and more expensive, is where the blades are geared to move in unison from one position to another. The freely pivoted blades work quite well and are generally less expensive but are not without some irritants. For example when the shaft stops, with the blades at or near the vertical position, the upper blade folds by virtue of gravity but the lower blade's weight makes it droop. This achieves only part of the hoped for drag reduction. Some racing yachts have bottom windows to view the propeller so that the shaft can be manually rotated to a position where both blades are folded to their inoperative position. It is not uncommon for a crew member to dive under a boat to place an elastic band around the blades to keep them folded together during a race. There is also the problem that should one blade stick and fail to open the resulting imbalance produces severe vibration when used.
Folding blade propellers, in which blades are interconnected to open and close in unison, eliminates some of the above disadvantages. While there may be other ways to achieve the interrelated movement the use of mating gears to do so is common. Here it might be mentioned that a folding blade propeller has two positions, one wherein the blades in the inoperative, or folded, position are essentially parallel to the axis of rotation of the propeller and in the other being the operative or open position where the blades are perpendicular to the axis of rotation.
In the known propellers with synchronized blades, each blade has a spur gear section concentric with its pivot axis to control the movement of the blades. The gears mesh with one another and as one blade rotates about its pivot mounting pin the other blade must open or close at the same rate. These gears are accurately machined to keep friction to a minimum. Foreign matter such as zebra mussels can impede the movement of the gears and limit the opening or closing of the blades.
Geared propellers normally are made either of bronze or cast iron. Cast iron propellers are less expensive than their bronze equivalent but have a serious drawback. Water near the surface usually contains a higher degree of dissolved oxygen. That tends to rust the mating faces of cast iron gear teeth. As rust forms, it is worn off with each folding, or unfolding of the blades, thus presenting a fresh new surface for further rusting. This combined rust-wear action quickly erodes the gear portion of cast iron propellers to the point that within a few years they fail to function and the propeller must be replaced. Badly rust-eroded gear teeth eventually skip or jam. Skipping causes non-synchronous motion. Jamming often locks one blade in the open position while the other remains folded. This creates severe imbalance. It can happen with no prior warning. The resulting vibration can make use of the engine impossible. In situations where use of an engine is essential, loss of power can render a vessel helpless and put it in severe danger.